Its a good question.
The shrouds on this model of a/c fit very poorly. Shockingly bad in fact. Rain water has no problem finding its way inside the enclosure. If all goes well it will drain out to the roof. If not ...
Since I store the trailer 1000 miles from where I live, I want to do everything possible to avoid a leak via the a/c opening.
I suppose it might provide sun protection for the plastics as well